1. Field of the Invention
The present invention relates to a control method and a control device of a molding machine and, more particularly, to a control method and a control device of a molding machine such as an electrically driven injection mold machine.
2. Description of the Related Art
A large part of an electric power consumed by an electrically driven injection molding machine is consumed by rotating equipments, that is, electric motors except for an electric power for a heater provided around a heating cylinder for melting a resin. There are various electric motors such as a motor for injection, a motor for screw rotation, a motor for mold clamping, a motor for an ejector, etc, which are incorporated into an electrically driven injection molding machine.
Here, in order to reduce an electric power consumption of a motor, what is necessary is just to reduce an electric current. That is, what is necessary is to reduce a force at a portion which applies a large force continuously. One of processes that needs a continuous large force is a pressure holding process. The pressure holding process is a process for applying a constant pressure to a screw so as to shape a molded article within a mold after a filling process for filling a melted resin into a mold by causing the screw to move forward within a heating cylinder by a motor for injection.
As an example of the pressure holding process, there is known a process which can set a holding pressure in at a plurality of stages (here, four stages) by an input device to a control device of an injection molding machine as shown in FIG. 1. Specifically, a pressure setting value and a keeping time of a holding pressure are set by an operator through the input device. As a result, the control device performs a pressure holding control to hold the setting value of the holding pressure set for each stage through a driver of a motor for injection.
As shown in FIG. 1, when a holding pressure is set up, a pressure actual result value of each actual stage changes as shown in FIG. 2. That is, the pressure actual result value changes with certain slope after a set time period has passed, and reaches a pressure setting value of a subsequent stage.
The following patent document 1 suggests a technique which can set the slope with a plurality of kinds.
Patent document 1): Japanese Laid-Open Patent Application No. 2001-293762 (first page, FIG. 3)
According to the suggestion in the Patent Document 1, especially when the pressure is decreasing, for example, when shifting from a pressure setting of a first stage to a pressure setting of a second stage, a control is made so that the pressure setting value of the second stage is reached after a time T1 with a delay of time ΔT although the pressure setting value of the second stage is reached at the time T1. This means that an electric power of a motor corresponding to an area indicated by dotted lines is consumed excessively.
However, with the shift from a hydraulic drive system to an electric drive system in recent injection molding machines, a reduction in a power consumption is has become a very important issue. Accordingly, the above-mentioned excessive power consumption is not preferable in a point of view of the reduction in power consumption.